energies

Article

The Influence of Eroded Blades on Wind TurbinePerformance Using Numerical Simulations

Matthias Schramm 1,2,* ID , Hamid Rahimi 1,2 ID , Bernhard Stoevesandt 1 ID and Kim Tangager 3

1 Fraunhofer Institute for Wind Energy and Energy System Technology, Küpkersweg 70,Oldenburg 26129, Germany; bernhard.stoevesandt@iwes.fraunhofer.de

2 ForWind, University of Oldenburg, Ammerländer Heerstr. 114-118, Oldenburg 26129, Germany;hamid.rahimi@uni-oldenburg.de

3 Blade Repair Solutions, Hedevej 25, Hadsund 9560, Denmark; kim.tangager@bladerepairsolutions.com* Correspondence: matthias.schramm@uni-oldenburg.de; Tel.: +49-441-798-5015

Received: 23 August 2017; Accepted: 11 September 2017; Published: 16 September 2017

Abstract: During their operation, wind turbine blades are eroded due to rain and hail, or theyare contaminated with insects. Since the relative inflow velocity is higher at the outer than at theinner part of the blades, erosion occurs mostly at the outer blade region. In order to prevent strongerosion, it is possible to install a leading edge protection, which can be applied to the blades afterthe initial installation, but changes the shape of the initial airfoil sections. It is unclear how thismodification influences the aerodynamic performance of the turbine. Hence, it is investigated in thiswork. The NREL 5 MW turbine is simulated with clean and eroded blades, which are compared tocoated blades equipped with leading edge protection. Aerodynamic polars are generated by meansof Computational Fluid Dynamics, and load calculations are conducted using the blade elementmomentum theory. The analysis in this work shows that, compared to clean rotor blades, the worseaerodynamic behaviour of strongly eroded blades can lead to power losses of 9%. In contrast, coatedblades only have a small impact on the turbine power of less than 1%.

Keywords: erosion; rotor blades; airfoil simulation; Computational Fluid Dynamics; Blade ElementMomentum theory

1. Introduction

Since wind turbines operate in the lower atmosphere, they are exposed to the local weather.Rain, hail or even insects lead to the erosion of rotor blades. Due to the higher rotational speed,this erosion mostly appears at the leading edges in the outer part of the blades. Although measurementsof the aerodynamic effect of erosion on real operating wind turbines are rare, it is known that erosionleads to higher drag and lower lift, especially at angles of attack (AoAs) near the stall.

Dalili et al. [1] reviewed the effects of surface roughness due to insects or ice on wind turbineperformance, but not much is said about the effect of leading edge erosion due to rain or hail.Corten and Veldkamp [2] showed within field experiments that increased roughness by insectcontamination can lead to power losses of more than 25%. It can be expected that strong erosion andresulting delamination lead to even higher power losses. Experiments with leading edge roughness onairfoils by Janiszewska et al. [3] showed a decrease in maximum lift of 25%, whereas the minimumdrag was increased by 60%. The stall angle was shifted slightly to smaller AoA.

Sareen et al. [4] investigated the effect of erosion on airfoils experimentally with varying degreesof erosion, where the measurements were conducted at Reynolds numbers between Re = 1 × 106 andRe = 1.85 × 106 in a low-turbulence subsonic wind tunnel. In the case of heavy erosion, the maximumlift was decreased by 17% compared to the lift of the clean airfoil, and the drag was increased upto 500%, although it should be noted that this increase did not refer to the minimum drag in the

Energies 2017, 10, 1420; doi:10.3390/en10091420 www.mdpi.com/journal/energies

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laminar drag bucket. The resulting polars of the experiments were used in numerical simulations ofa 2.5 MW turbine based on blade element momentum theory (BEM), and they concluded that heavyerosion could lead to a loss in annual energy production (AEP) of approximately 25%. Note that thesimulations were done using the same level of erosion along the full blades, which is probably not thecase for real turbines due to varying rotational speeds.

In order to avoid or prevent erosion, it is possible to use special leading edge protections.Weigel [5] investigated a protection system for helicopter blades, which are eroded by rain or sand.He showed that the protection used could reduce the effect of erosion. Still, helicopter blades aredifferent from wind turbines due to different flow behaviour and rotor speeds. Additionally, thematerials of helicopter blades and possible coatings are different from those of wind turbines.

In contrast, Giguere and Selig [6] experimentally investigated the effect of tapes on wind turbineairfoils. The wind tunnel measurements were conducted at low Reynolds numbers from Re = 150,000 toRe = 500,000 and airfoils which are not used on bigger turbines. The power losses due to the used tapeswere less than 2%, which is small compared to the losses with eroded airfoils. Still, it is not known howmuch a leading edge protection changes the wind turbine performance for modern multi-MW turbinescompared to eroded airfoil shapes. This is the focus of this work, and in order to be able to reach thehigh Reynolds numbers of modern wind turbines, the investigation is done purely numerically.

Fernandez-Gamiz et al. [7] investigated the effect of microtabs and could show that windturbine power can be increased by using these add-ons. They computed airfoil polars by means ofComputational Fluid Dynamics (CFD) and used these polars in BEM computations to estimate thepower output of a 5 MW turbine. A similar approach is followed in this work.

Corsini et al. [8] report a numerical study of rain erosion on a 6 MW turbine and comparedstandard and aerodynamically optimized blades with each other. They simulated erosion bya steady-state Euler-Lagrangian approach with droplet tracking using CFD, but they do not report anydifference in total power or AEP between eroded and clean blades.

Thus, the following work shall give a numerical estimation of the expectable losses due toerosion, and shall compare the wind turbine performance to coated blades equipped with leadingedge protection. The selected turbine is the NREL 5 MW research turbine [9], which is often used incomparative research studies [10,11]. It is a three-bladed horizontal axis upwind turbine with a ratedpower of 5 MW at a wind speed of 11.4 m/s and a rotational speed of 12.1 rpm. The airfoil in theouter region of the turbine is a slightly modified version of the NACA 64-618, called “NACA64_A17”in the NREL reports. Since the coordinates of this modified airfoil are not available for the authors,the NACA 64-618 was used instead.

After a validation of the numerical method, aerodynamic polars of clean, eroded, and coatedairfoils were generated by means of CFD. Then, these polars were used in BEM calculations in order toestimate power and loads as well as the AEP for an exemplary site.

2. Numerical Methods

In this section, the numerical methods used in this work are described. The aerodynamiccoefficients are computed by CFD, which is validated for the basic approaches. The generated polarsare later used in turbine load calculations, which are based on the BEM theory.

2.1. Simulation of Airfoil Aerodynamics

The aerodynamic coefficients of the airfoils in this work were computed by means of CFD, wherethe steady-state Reynolds-averaged Navier-Stokes (RANS) equations were solved. The computationswere conducted with OpenFOAM-2.3.0 [12], which is an open-source CFD code written in C++.The general validity of the code for wind turbine applications was shown by Herráez et al. [13].

Ferrer and Munduate [14] investigated roughness effects on an S814 airfoil by NREL andcompared experiments to two-dimensional simulations. They showed that using the panel codeXFoil [15,16] with boundary layer tripping is not sufficient for modelling rough airfoils, and instead

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CFD is necessary, where rough wall functions and the k-ω-SST model were used. Although it is oftendone, dirty conditions can be modelled only to some extent by tripped XFoil computations, because“roughness is a complicated phenomenon that cannot be reduced only to a change in the transitionpoint location” [14]. Thus, as suggested, CFD is used in this work.

In order to close the RANS equations, the k-ω-SST model was used [17], where a further equationfor the intermittency γ was added in order to consider transition from laminar to turbulent flow [18].Two-dimensional hexahedral meshes (O-structure) were used with 512 cells on the airfoil and 256 cellsin the airfoil’s normal direction, leading to approx. 130, 000 cells in total. This mesh resolution wasmotivated by the mesh studies of the following Section 2.1.1. In order to completely resolve the wallboundary flow, a dimensionless wall distance of y+ < 1 was used, and the radius of the domainwas 80 · c, where c is the chord length of the airfoil. Figure 1 shows an examplary mesh of theNACA 64-618 (complete domain and close-up view).

Figure 1. Complete domain and close-up view of the mesh for a NACA 64-618.

For simulating rough walls, wall functions were used, wherein the coefficients of the law of thewall were modified [19]. By the use of such wall functions, a higher dimensionless wall distanceof y+ ≈ 30 was used, and the number of cells in the radial direction was reduced to 192.

In order to ensure that the numerical predictions of the following airfoil simulations can be usedfor the later load calculations of a wind turbine, the CFD is validated in this section. First a validationof a DU 96-W-180 at Re = 1.5 × 106 is shown for clean, rough, and strongly eroded configurations,then a validation of a clean NACA 64-618 at Re = 9 × 106 is presented.

2.1.1. Validation of DU 96-W-180 at Re = 1.5 × 106

In experiments, surface roughness is sometimes modelled by boundary layer tripping, which isthen simulated with a fixed transition location in numerical methods [20]. In fact, real roughness dueto erosion leads to stronger effects than the ones of a tripped boundary layer [14], and thus a validationof numerical methods is done here against experiments having strong roughness.

Sareen et al. [4] experimentally investigated the effect of erosion with different strength ona DU 96-W-180 wind turbine airfoil at Reynolds number of Re = 1.5 × 106 in a low-turbulencesubsonic wind tunnel. The clean airfoil is simulated here with a model for natural transition (y+ < 1).The strongest erosion in the work of Sareen et al. (“Type C, Stage 5”) is compared here to simulationsusing rough wall functions (y+ ≈ 30) and a fully-turbulent flow model. Stronger erosion leads todelamination and a change in the leading edge shape, which is shown in Figure 2 on a real windturbine blade. The turbine operates in an area with many insects (Australia), and the picture is takenapprox. one year after installation. Turbines in other areas have a similar amount of erosion aftera few years.

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Figure 2. Erosion on a real rotor blade on a near-shore wind turbine after approx. one year of operation(courtesy of Blade Repair Solutions).

Such strong erosion can no longer be modelled by rough wall functions and thus a differentleading edge shape needs to be simulated. The model used in this validation is shown in Figure 3,which is simulated with a fully-turbulent flow model without transition, but resolved boundarylayer (y+ < 1).

Figure 3. Sketch and close-up view of erosion modelled by a deformed leading edge as it is used inthe two-dimensional CFD simulations (erosion model in grey and original DU 96-W-180 in blue).

Mesh studies were performed for all different levels of roughness, where in each study threedifferent meshes with the same domain size and the same y+-values were used. Going to a finer mesh,the number of cells are doubled in the radial direction and along the airfoil. In cases with rough wallfunctions, a higher dimensionless wall distance and accordingly less cells in the radial direction wereused. The number of cells used in the mesh studies are shown in Table 1.

Table 1. Number of cells used in the mesh studies.

Cells Around AirfoilCells in Radial Direction

for y+ < 1Cells in Radial Direction

for y+ ≈ 30

coarse meshes 256 128 96medium meshes 512 256 192

fine meshes 1024 512 384

The results of the mesh study at an AoA of 4◦ are shown in Table 2, where the relative differencesδli f t, δdrag are calculated based on the lift and drag coefficients resulting from the fine meshes. In mostcases, a clear improvement of the medium mesh compared to the coarse mesh can be seen. Since theeroded airfoil is simulated with a different shape as shown in Figure 3, a higher resolution of the airfoilshape (i.e., more cells along the airfoil) leads to a different representation of the kink at the leadingedge. This means that with different meshes, slightly different geometries are simulated, which showsthe limitation of such a mesh study and explains the higher deviation of the drag coefficient for theeroded airfoil (δdrag > 2%). Nevertheless, the medium meshes are considered to deliver sufficientlyconverged results at moderate computing times. They were used for the following simulations.

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Table 2. Relative differences δli f t, δdrag in (%) of lift and drag coefficients compared to the coefficientsresulting from the fine meshes (differences are presented as absolute values).

Clean Airfoil(y+ < 1)

Rough Airfoil(y+ ≈ 30)

Eroded Airfoil(y+ < 1)

δli f t δdrag δli f t δdrag δli f t δdrag

coarse meshes 0.074 6.1 0.96 1.3 4.2 18.0medium meshes 0.31 0.74 0.13 0.28 0.063 2.2

fine meshes 0.0 0.0 0.0 0.0 0.0 0.0

After the demonstration of the mesh independency, simulations were compared to experiments.Lift polars show the lift coefficient cl against the AoA. Drag polars show the lift coefficient cl againstthe drag coefficient cd. The resulting lift and drag polars using different levels of roughness are shownin Figure 4.

Figure 4. Validation of numerical results by Computational Fluid Dynamics (CFD) against wind tunnelmeasurements of a DU 96-W-180 at Re = 1.5 × 106 (“clean CFD” refers to resolved boundary layer withtransition model, whereas “rough CFD” refers to the use of wall functions with fully-turbulent flowmodel and “eroded CFD” refers to a changed leading edge shape with resolved boundary layer andfully-turbulent flow model).

In general, the clean and rough simulations follow the trend of the experiments, but deviations canbe seen in the stall, where the lift and drag coefficients are over-predicted by the CFD. This is a commonproblem of most turbulence models, and results from the two-dimensional steady simulations,but three-dimensional measurements of an unsteady flow. Furthermore, by the concept of rough wallfunctions, it is not possible to include the complete details of erosion, since the approximations in themodelling approach contain some limitations; for example, the airfoil shape is not changed and thesimulated roughness is evenly distributed, which may not be the case in reality. Compared to cleanand rough simulations, the deformed shape due to strong erosion showed an even stronger decreasein lift and increase in drag near the stall. This was not seen in the experiments, but there even thestrongest erosion type (“Type C, Stage 5” [4]) did not lead to a deformation as it happens on real windturbines (shown in Figure 2). With the presented methods of modelling clean, rough, and eroded

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airfoils, a broad range of surfaces can be simulated, and in order to investigate their influence on theturbine performance the numerical set-ups will be used in the later simulations as well.

2.1.2. Validation of NACA 64-618 at Re = 9 × 106

The NACA 64-618 is an airfoil with a relative thickness of 18%, which was initially developed foruse in the aviation industry, but can also be used in the outer region of wind turbines (e.g., the NationalRenewable Energy Laboratory (NREL) 5 MW turbine in this work). The experimental results of theNACA 64-618 are taken from Timmer [21], who compared some work from Abbott et al. [22,23] withhis own simulated results. The measurements were done in the Langley low-turbulence pressure windtunnel (LTPT) at a Reynolds number of Re = 9 × 106 and the airfoil was measured in clean conditions.

Figure 5 shows the lift and drag polar of the numerical and experimental results. The generalagreement between simulations and measurements is good, although some deviations can be seen.The numerical lift in the linear range is a little higher than in the measurements, and the drag is nearlyconstant instead of showing the laminar drag bucket of the experiments. Since the focus of this workis a comparative study, the validation results are considered to be good enough for the followinginvestigations at high Reynolds numbers.

Figure 5. Validation of numerical results by CFD against wind tunnel measurements of a NACA 64-618at Re = 9 × 106 (“clean CFD” refers to resolved boundary layer with transition model).

2.2. Load Calculations

The aerodynamic coefficients computed by means of CFD were used in turbine load calculations,which were performed by the aeroelastic BEM code FAST v8 [24]. FAST is developed by the NationalRenewable Energy Laboratory (NREL), and is established as a comprehensive aeroelastic framework,capable of predicting both the extreme and fatigue loads of two- or three-bladed horizontal-axis windturbines. In this study, the effects of erosion and leading edge protection are investigated for theNREL 5 MW turbine [9], which is one of the certified tests in the FAST package.

Power, thrust force, and root bending moments are computed within FAST following the equationsin the AeroDyn Theory Manual [25]. Vertical wind shear is calculated using a power-law, which is alsopart of the IEC Standards [26]. The tower is not modelled in order to isolate the effects of the differentairfoil polars. An extrapolation for higher AoA is done by FAST with the method of Viterna [27],and the dynamic stall model of Beddoes and Leishman [28] is used. Air viscosity is selected to be1.464×10−5 m2/s with the air density of 1.225 kg/m3. The pitch angles were kept constant betweendifferent cases in order to have a clearer performance comparison of the different airfoils.

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3. Airfoil Polars with and without Leading Edge Protection

After the validation of CFD methods, the airfoils with different leading edge shapes weresimulated with the same computational set-up described in Section 2. Due to the different Reynoldsnumbers, the wall distance changed, but the dimensionless wall distance remained the same.The airfoils were simulated with clean and rough configurations, whereas a strongly eroded airfoilwas simulated by a changed leading edge shape. The results were compared to a coated leading edge.Two different Reynolds numbers (Re = 7.5 × 106, Re = 11.5 × 106) were used due to the varying chorddistribution and rotational speeds along the radius of the wind turbine blade. In order to limit thecomputational effort, the airfoil polars—which will be used in the following BEM simulations—werenot generated for the complete range of 360◦, because this is usually not required for normal operatingturbines. Some cases (e.g., standstill) require aerodynamic coefficients at AoA of 90◦, but this exceedsthe interest of this work. Hence, the polars are generated only for AoA between −24◦ and 28◦ withsteps of 2◦, which covers the range of interest in this work.

3.1. Eroded Leading Edge

As previously shown in Figure 2, strong erosion can delaminate and deform the leading edge ofrotor blades. A simplified two-dimensional model of an eroded airfoil is shown in Figure 6, comparedto the initial NACA 64-618.

Figure 6. Sketch and close-up view of erosion modelled by a deformed leading edge as it is used inthe two-dimensional CFD simulations (model for strong erosion in grey and original NACA 64-618in blue).

The simulation results of such a shape are compared with the original airfoil under clean and roughconditions in Figure 7, where the lift and drag coefficients are shown against the AoA. Compared to theclean airfoil, the configuration with rough wall functions leads to a decrease in lift near the stall and ageneral increase in drag. When using the eroded shape, the lift is even further decreased. Additionally,the drag in the stall is further increased, although the minimum drag is smaller than at rough conditions,which could result from the particular leading edge shape modelled without additional rough wallfunction. In summary, rough and eroded airfoils clearly have a worse aerodynamic performance thanthe clean airfoil.

Figure 7. Cont.

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Figure 7. Numerical lift and drag coefficients of clean, rough, and eroded airfoils at Re = 7.5 × 106 andRe = 11.5 × 106 (“clean” refers to resolved boundary layer with transition model, whereas “rough”refers to the use of wall functions with fully-turbulent flow model and “eroded” refers to a changedleading edge shape with resolved boundary layer and fully-turbulent flow model).

3.2. Leading Edge Protection

In order to prevent erosion or in order to repair eroded wind turbine blades, it is possible tomount a special leading edge protection, which could also be applied on the blades after the initialinstallation. The coating can be applied on turbines without removing the blades, which reduces thecosts compared to other techniques, since the shut-down time is shorter and the general maintenanceeffort is smaller.

Figure 8 shows the model of a leading edge protection on the NACA 64-618. The protectionhas a total length of 20% and ends at approx. 7.6% of the chord. At the leading edge center, it hasa height of approx. 0.2% of the chord and approx. 0.1% at the ends of the protection. With thesedimensions, the difference in shape may be in the order of the production tolerances of some rotorblades. Thus, it is expected that the coating has a small effect on the turbine performance.

Figure 8. Sketch and close-up view of leading edge model with protection as it is used in thetwo-dimensional CFD simulations (model with protection in grey and original NACA 64-618 inblue).

The resulting lift and drag coefficients of the clean and coated airfoil are shown in Figure 9 againstthe AoA. Compared to the initial airfoil, the protection reduces the lift near the stall angles and the dragis increased at these, but also at smaller angles. The differences are on the order of approx. 8%, but theyare still clearly much smaller than in the rough or strongly eroded airfoils of the previous section.

Figure 9. Cont.

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Figure 9. Numerical lift and drag coefficients cl , cd of clean and coated airfoils at Re = 7.5 × 106 andRe = 11.5 × 106 (“clean” refers to resolved boundary layer with transition model, whereas “coated”refers to a changed leading edge shape with resolved boundary layer and fully-turbulent flow model).

4. Simulation of Wind Turbines—Load Calculations

In this section, the aerodynamic behaviour of a turbine using clean, rough, and eroded airfoils iscompared to a turbine with leading edge protection. The load calculations were performed by theaeroelastic BEM code FAST v8 [24] as mentioned in Section 2. In this study, the effects of erosion andleading edge protection are investigated for the NREL 5 MW turbine [9], which is one of the certifiedtests in the FAST package. Table 3 lists the basic characteristics of this numerical reference turbine.

Table 3. Basic characteristics of the NREL 5 MW reference turbine [9].

Rated power 5 MWRotor diameter 126 mAxial induction 0.25

Cut-In, Rated Rotor Speed 6.9 rpm, 12.1 rpmOverhang, Shaft Tilt, Precone 5 m, 5◦, 2.5◦

Tip speed 80 m/sHub height 90 m

Most of the airfoil polars—namely, polars of the DU 40-A17, DU 35-A17, DU 30-A17, DU 25-A17,DU 21-A17 which are used in this contribution—are mainly from the FAST package [9]. Only thepolars of the outer part of the blade from the NACA 64-618 were computed by means of CFD asexplained in Section 3. Due to a varying chord distribution and rotational speed, the Reynolds numbervaries along the radius of the blade, and the airfoil polars were computed for two Reynolds numbersonly; i.e., the minimum and maximum Reynolds number of the NACA 64-618 at rated conditions(inflow wind speed of vin f low = 11.4 m/s and rotational speed of n = 12.1 rpm). This leads to aReynolds number of Re = 7.5 × 106 at a radial position of 61.63 m with a chord length of 1.42 m and toa Reynolds number of Re = 11.5 × 106 at a radial position of 44.55 m with a chord length of 3.01 m.The polars by CFD computations were interpolated for other Reynolds numbers at intermediatesections.

4.1. Aerodynamic Power, Thrust, and Root Bending Moments

Figure 10 shows the aerodynamic power and the thrust curve for the rotor using the differentairfoil polars from Section 3. Compared to the clean blade, the eroded airfoils led to a decrease inaerodynamic power for all wind velocities. In particular, above rated wind speed of vin f low = 11.4 m/s,a power loss of 9% can be seen. A similar trend can be observed when comparing rough to cleanblades. In contrast, the coated blade with leading edge protection reduces the power only slightly byapprox. 1% compared to the clean blade.

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The eroded blade clearly led to lower thrust near the rated wind speed, whereas the rough bladeonly slightly decreased the thrust. The difference between clean and coated blades was negligiblysmall (below approx. 2%). Hence, it can be concluded that, firstly, leading edge erosion will not lead toincrease in thrust, and secondly the airfoil section with leading edge protection led to similar thrust asa clean blade.

Figure 10. Power and thrust curve of the NREL 5 MW turbine using different sets of polars for theNACA 64-618 airfoil.

For the rated case at 11.4 m/s, Table 4 lists a more detailed comparison of the aerodynamic powerand rotor thrust for all sets of airfoil polars with regard to the clean blade, which is used as a referencein the comparison. It can be observed that the eroded and rough wind turbine blades could causeup to 9% loss in power production. Hence, the blade erosion has a significant effect on the windturbine performance at rated conditions. In contrast, the use of a leading edge protection could limitthe deviation, and the coated blade led to a power loss of less than 1% (at this wind speed), which mayeven be below the numerical errors.

The rotor thrust in Table 4 is less strongly affected by the blade erosion as the power output.This is related to the fact that surface roughness and blade erosion mainly have an effect on the drag.Since the lift polars do not differ significantly within the linear region, the thrust force is not as severelyaffected as the power.

Table 4. Comparison of the aerodynamic power and rotor thrust using different sets of polars at ratedconditions (reference is the clean blade).

Difference in (%) to Clean Blade

Clean Eroded Rough Coated Eroded Rough Coated

power in (kW) 5540 5040 5170 5500 −9.00 −6.70 −0.80thrust in (kN) 873 816 851 859 −6.50 −2.50 −1.60

Figure 11 shows the edgewise and flapwise root bending moments over the wind speed. At higherwind speeds close to the rated wind speed of 11.4 m/s and above, the eroded and rough airfoil sectionsled to smaller edgewise moments compared to the clean blade. The differences between clean andcoated blades were the smallest.

The flapwise root bending moments were generally similar to the thrust curves in Figure 10,and the eroded airfoil led to smaller moments. The rough airfoil section led to slightly smaller flapwisemoments, and again, the differences between clean and coated blades were the smallest.

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Figure 11. Edgewise and flapwise moments at the root for one blade of the NREL 5 MW turbine usingdifferent sets of polars for the NACA 64-618 airfoil.

Table 5 lists the results of blade root bending moments for rated conditions in a similar format asin Table 4. In terms of blade root bending moments, a trend similar to the aerodynamic thrust outputcan be observed. The loads at the blade root of the eroded and rough rotor blades show significantdifferences compared to the rotor blade equipped with erosion protection. Along with the decreasedpower output and the decreased rotor thrust, the root bending moments are also clearly reduced.

Table 5. Comparison of the edgewise and flapwise root bending moments using different sets of polarsat rated conditions (reference is the clean blade).

Difference in (%) to Clean Blade

Clean Eroded Rough Coated Eroded Rough Coated

edgewise moment in (kNm) 1210 1080 1120 1170 −11.00 −7.50 −3.30flapwise moment in (kNm) 11,700 10,800 11,300 11,500 −7.90 −3.10 −2.00

It should be noted that the NREL 5 MW turbine operates at AoA in the range of 4◦ and 5◦ at ratedconditions. According to Figure 7, the deviations at this range between clean and eroded polars arenot more than 15%. However, it can be clearly seen that increasing the operational AoA higher than 6◦

will lead to higher power losses and also more deviation in the loads, which may happen in turbineswith a different design.

Figure 12 shows the aerodynamic power and thrust of the NREL 5 MW turbine at rated conditionfor different yaw angles from 0◦ to 30◦. As in the case under axial condition, the eroded and roughblades led to lower power and thrust compared to clean and coated blades. The differences seem to beconstant when the yaw angle changes. For the clean and eroded blade, the corresponding differencesstay within a range of approx. 9%.

Figure 12. Rotor power and thrust against yaw angle of the NREL 5 MW turbine at rated conditionusing different sets of polars for the NACA 64-618 airfoil.

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4.2. Energy Production

To judge the effect of the developed erosion protection, it is not sufficient to evaluate the powercurve of a wind turbine only. Much more important in wind energy is the AEP of a wind turbinewith coated blades in order to estimate the return on investment. To give an estimation of the effecton the AEP, the generated results from Section 4.1 were used to evaluate the AEP based on a Weibulldistribution [29] for the wind velocities over one year. In this study, the wind speed distributionmeasured with a met mast at Risø was used [30]. Figure 13 illustrates the measured wind speeddistribution of the site at Risø for different heights. The wind speed was measured for ten years atheights of 76 m and 117 m. Since the hub height of the NREL 5 MW wind turbine is 90 m, the Weibullparameters were linearly interpolated in order to estimate the wind distribution for 90 m. Hence,the resulting Weibull parameters are the shape parameter k = 2.23 and the scale parameter A = 8.57 m/s,which represents the characteristic mean wind speed.

Figure 13. Wind speed distribution at two different heights of the Risø met mast [30] and interpolatedwind speed distribution at 90 m.

Using the Weibull distribution and the power curves from the previous section, the AEP can beeasily calculated.

Table 6 shows the AEP for the NREL 5 MW reference turbine for the exemplary site using differentsets of polars. For the sake of simplicity, it is assumed that the wind turbine has an availability of100%. It can be observed that the difference in aerodynamic power output, evaluated in Section 4.1,translates into a significant difference in the AEP for the different blades.

Table 6. Comparison of the annual energy production (AEP) using different blades.

Blade Type AEP(MWh/Year)

Difference in AEP(%)

clean 16,800 0eroded 15,600 −7.10rough 15,400 −8.30coated 16,500 −1.80

Using rough or eroded blades resulted in a loss of approx. 8% in AEP, and although the lowestpower at rated conditions was produced with the eroded blade (see Table 4), the AEP of the roughblade was the lowest in this comparison. This results from the slightly higher power output of theeroded blade at low and high wind speeds, which adds up to a slightly higher AEP compared tothe rough blade. However, the wind turbine equipped with the coated blades using a leading edgeprotection had the lowest losses compared to the undamaged clean wind turbine blade.

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The losses can add up to a considerable amount of financial means if a lifetime of 20 years isconsidered for a wind turbine. Table 7 provides an overview of the loss in financial means for oneyear. Since the price per kWh depends strongly on the current situation on the energy market and/orexisting contracts based on feed-in rates, in this study two different, exemplary prices are considered(3 ct/kWh and 5 ct/kWh).

Table 7. Comparison of the yearly loss of revenue using different blades for two exemplary energyprices (losses are compared to the yearly revenue of the clean turbine blades).

Yearly Loss of Revenue

Blade Type with 3 ct/kWh(EUR/Year)

with 5 ct/kWh(EUR/Year)

clean 0 0eroded −37,500 −62,500rough −42,000 −70,000coated −9000 −15,000

Significant differences can be observed, and the use of a leading edge protection seems to bebeneficial compared to rough or eroded blades.

Still, this is a rough estimation, and strongly depends on various aspects such as site and installedturbine. A careful trade-off must be done between turbine repair and installation of a leading edgeprotection, where a loss of revenue appears as well.

5. Conclusions

In order to prevent wind turbine blades from strong erosion, it is proposed to use an erosionprotection for the leading edge of rotor blades. In this work, the aerodynamic behaviour of a clean,rough and eroded blade of the NREL 5 MW wind turbine was compared to a blade equipped with theproposed leading edge protection. Since mainly the outer parts of the blades are exposed to erosiondue to the higher rotational velocity, the NACA 64-618 was selected as the airfoil of interest. To includethe effect of erosion and roughness, new polars were generated using modified airfoil geometriesand surface roughness by means of CFD. The aeroelastic simulation framework FAST was used tosimulate the effect of different airfoil polars on the performance of a wind turbine. The aeroelastic loadcalculations showed that the eroded blades can lead to a drop in power output of up to 9%. This directlytranslates into a loss in AEP of approx. 8%, which reduces the financial income of a turbine due toerosion.

In contrast, it was shown that the blade equipped with leading edge protection performedsimilarly to the clean, undamaged blade. The power output was only slightly decreased and themoney loss was much smaller than that of a turbine with eroded blades. Since the NREL 5 MW turbineoperates at relatively low AoA (approx. 4◦), an increase in the AoA due to yaw or pitch could leadto even stronger power losses. A significant increase in loads could not be observed in any of theinvestigated cases compared to the undamaged turbine with clean blades. This also means that thecoated airfoil section did not increase the loads of the investigated turbine. From the presented results,it can be concluded that the proposed leading edge protection seems to be a good option when theblades are eroded. Still, it must be noted that several assumptions were made during this investigation(e.g., 2D numerical polars, met mast data from one site only, use of a research turbine). The resultsshould be handled with care, and they can only give an indication of the phenomena in the real world.Moreover, the loss in power and money always depends on a particular site and turbine type.

Nevertheless, the results showed a considerable advantage of blades with leading edge protectionover eroded shapes. From a purely technical point of view, it is proposed to use such protectiondevices, since the blades are often eroded after a fraction of their life time. Economically, the pay-offdepends on the frequency and price of blade repair, and each operator should carefully calculate the

Energies 2017, 10, 1420 14 of 15

trade-off between repair, leading edge protection, or further use of eroded blades. Economical studiescould be done for a variety of sites and turbines. Besides, further investigations may be done eitherwith more numerical details as well as wind tunnel measurements or on real operating wind turbines.

Acknowledgments: The authors also would like to thank Martin Dörenkämper (Fraunhofer IWES) for providingthe information about the Weibull distribution at the exemplary site and Bastian Dose (Fraunhofer IWES) forsupport in evaluating the results.

Author Contributions: Matthias Schramm performed the CFD simulations and wrote the paper; Hamid Rahimiperformed the BEM calculations as well as the calculations of the energy output and wrote the paper;Bernhard Stoevesandt supervised the work and analyzed the data; Kim Tangager contributed to the generalinterpretation of the results.

Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:

AEP Annual Energy ProductionAoA Angle of AttackBEM Blade Element MomentumCFD Computational Fluid DynamicsNREL National Renewable Energy LaboratoryOpenFOAM Open Field Operation And ManipulationRANS Reynolds-Averaged Navier–Stokes

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